Once Again, Don’t Believe Facebook When It Comes to Science!

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Cyclone Larry near northeast Australia (left) and Hurricane Hernan near Mexico (right). Notice that the rotation in the Southern hemisphere is clockwise, and the rotation in the Northern Hemisphere is counterclockwise.

(Images Courtesy of NASA/GSFC, by Jeff Schmaltz and Jacques Descloitres)

A few months ago, I posted an article about how you should not believe Facebook memes and videos when it comes to science. Of course, I have seen more scientific nonsense on Facebook, but lately the following video keeps popping up on my feed:

When I see it, I generally comment that the video is a clever con. It is based on a scientific concept that is quite true, but it does not apply to situations that involve short distances. Most of my Facebook “friends” express appreciation for me pointing out the error, but one of them was adamant for a while that the video shows a real truth. In my efforts to educate him, I ended up finding a really nice video resource, which I will share after my long-winded statements on this issue.

The picture at the top of the post illustrates a general trend in nature. Hurricanes and cyclones in the Southern Hemisphere rotate in a clockwise fashion, while hurricanes and cyclones in the Northern Hemisphere rotate in a counterclockwise fashion. This is because of the Coriolis effect, which is a consequence of the earth’s shape and its rotation.

The earth is a sphere, and it makes a full rotation every 24 hours (not exactly, but that’s okay). Since all parts of the earth have to return to where they were 24 hours previously, the land at different latitudes actually travels at different speeds (relative to the center of the earth). The land at the equator has to travel farther to get back to its original position than the land near the poles, so the land is actually moving faster (relative to the center of the earth) at the equator than it is at the poles. As objects (or gases) rise from the ground, they have the speed that the land has, so objects moving in the air near the equator are moving with the land faster than objects moving in the air far from the equator.

Because of this, the path of things that travel away from the equator appears to bend. If I fire a missile north from the equator, I am giving it a northern velocity, but it also gets an eastern velocity because the land is moving east with the earth’s rotation. At the equator, that eastern velocity is high, so the missile travels north because of the velocity I gave it, but also east because of the high velocity the land gave it. Once it is in the air, the land doesn’t push it anymore, so it continues to travel with a high eastern velocity. The land underneath the missile travels east more slowly the farther north the missile goes, so the missile ends up “outrunning” the land, and its path bends east. If you have a hard time visualizing that, here is a good website that has a great animation.

If I now shoot the missile south, the same thing happens. Its path bends east, because it has a faster eastward velocity than the land over which it is traveling as it heads south. So both missiles bend east. However, when I am standing at the equator and looking north, east is to my right, so I see the missile I shot north bending to the right. When I stand at the equator and look south, east is to my left, so I see the missile bending to the left. Now look at the storms above. Notice the clouds in the Southern Hemisphere (Larry) bend left from the center of the cyclone, while the clouds in the Northern Hemisphere (Hernan) bend right from the center of the hurricane.

So tropical storms rotate clockwise in the Southern Hemisphere and counterclockwise in the Northern Hemisphere. That’s what leads to the myth in the video shown above. If tropical storms do it, then water in a basin should do it, right? Wrong! This effect only works when the distances are long enough for there to be a noticeable difference in the speed of the land. A basin doesn’t cover enough distance to have a noticeable difference in the speed at which the land is traveling.

In the end, a basin drains clockwise or counterclockwise depending on its shape and how the water is introduced into the basin. It has nothing to do with the Coriolis effect. That leads me to the nice video I found refuting the nonsense in the video above. The guy in the video used the same sink in both the Northern Hemisphere and the Southern Hemisphere. He also poured water into it in the same way each time. As you can see, it drains in the same direction in both hemispheres. You can also see that it is very dependent on how the water is initially poured in:

If you see the first video pop up in your Facebook feed, please leave the second video in the comments! It’s address is https://youtu.be/pdMZjssrAlk.